there is a fundamental difference between a *chart* and a *graph* or diagram.

A chart is a map of some data (like a city map, but for mass-data). It is a graphical visualization of tabular data. Charts are used for statistical purposes. Charts may be helpful to make mass data more understandable.

A graph is a graphical representation of a relationship between some objects or concepts. (In other words: A graph is a drawing that explains how something works or behaves.)

It is a common property of human languages, that terms get mixed, so you will find the word ‘chart’ in classical graph types, like ‘flow-chart’. Nonetheless, by sticking to the definitions above, it is easy to see that a flowchart is no chart at all – its a graph.

Taking this into consideration, a Sankey diagram can be considered both, a Sankey chart and a Sankey diagram. The quantities represented by the magnitude of the flow could also be shown as tabular data, the direction of the flow, given by the arrow orientation between two processes indicates a ‘from-to’-relationship.

A scan of one of the first – if not THE first ever – published Sankey diagrams has now been added to the Dutch and German Wikipedia articles. Actually I had always wanted to get hold of a digital version of this this energy efficiency diagram published by Captain Henry R. Sankey in 1898 in the Minutes of Proceedings of The Institution of Civil Engineers. Vol. CXXXIV, Session 1897-98. Part IV.

I have been asked whether ‘Grassmann Diagrams’ are the same as ‘Sankey Diagrams’, or what distinguishes them from Sankey diagrams. Frankly speaking, I only came across Grassmann Diagrams one or two years ago, and I hadn’t heard (or had I overheard?) this term during my studies. So here is a short summary of what I found out about this special type of diagram.

Grassmann diagrams are usually referred to as ‘exergy diagrams’. Exergy, in thermodynamics, are being “defined as a measure of the actual potential of a system to do work” (see Wikipedia entry), or the maximum amount of work that can be extracted from a system. (For those who are looking for a well-written introductory article on exergy, I recommend the first chapters of this one by Wall and Gong, which also shows links to LCA, economics and desalination).

Coming back to Sankey diagrams, they were in the very first place used to show the energy balance, or energy efficiency of a machine or a system. (Today, however, the use of Sankey diagrams has been extended beyond displaying energy flows, and they are also used for any kind of material flows, CO2 emission, value flows, persons, cars, pig halves, and the like).

Thus the difference between Grassmann and Sankey diagrams is mainly that the first depict exergy, the latter energy. Taking this, it is understandable that the width of the flow gets less at each stage, while in Sankey diagrams the width of the arrow at a process (transformation, machine) should be maintained, as energy is only being transformed, but never being consumed (First Law of Thermodynamics).

Let’s forget about the semantics and their primary use for a second, and look primarily to the visualization aspect of both diagram types. Then, in a more general perception of Sankey diagrams as flow diagrams that display arrow widths proportionally to the flow quantities, Grassmann diagrams could be understood as a special subset of Sankey diagrams. Indeed, some authors refer to them Sankey-Grassmann diagrams, or as an adaptation of Sankey diagrams, or as the counterpart to Sankey diagrams.